Tuesday is the 13th episode of Season 4 and the 79th episode overall of the show Alias. When a mission goes wrong Sydney is attacked and placed in serious danger. Meanwhile, Dixon is exposed to a deadly pathogen released inside APO which is currently in lockdown due to the toxin and Marshall is the only one who can save Sydney. Only with com link help from APO Marshall has to do a very important mission before it's too late.

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At an upscale nightclub in Havana, Cuba, Sydney invites a man to dance. It's her informant, and he tells her that the Third Faction is planning a large-scale attack on a civilian target. He also tells Sydney where a hard drive containing more information could be found, which Dixon soon retrieves. They're being watched, however.

As a car takes Sydney to the airport, she flirts with Vaughn over the phone, with Vaughn promising her a slow dance. Suddenly, attackers shoot Sydney's driver and she's knocked out as her car crashes.

Dixon arrives at APO with the hard-drive. Since Marshall is late, another agent is tasked with the decryption. When the drive is plugged in, it sprays a toxin in the office. Dixon goes into a seizure and although he's injected with an antidote, the office is locked down.

As Marshall arrives in the parking lot, he phones Sloane to tell him that the firewall networks are still secure. Sloane asks Marshall to stay on the line as they call Sydney to confirm her flight.

The ringing phone wakes Sydney. To her horror, she realizes she's been buried alive, the dead informant beside her. Sydney calls back and Jack advises her to stay calm - but nobody can leave the office for 36 hours. Marshall volunteers to go to Cuba and track Sydney's cell phone with a Radio Frequency locator.

While Dixon recuperates, Sloane tells the rest of the team that the Third Faction has enough toxin to infect an entire city. Despite their predicament, they must still discover the target city and prevent the bomb from detonating.

Marshall arrives in Havana and creates his makeshift RF Locator. Sydney calls the office and helps them identify Ulrich Kottor, a significant figure in the Third Faction, by taking a photo of the dead informant and sending it thru her phone. APO links him to Ulrich.
Marshall arrives in the cemetery where Sydney is buried. Sydney's phone dies and Marshall realizes there are too many fresh graves for him to search. He asks Vaughn to direct a satellite over Marshall's position and change the view to thermal. In this way, Vaughn and Marshall locate Sydney.

Marshall digs frantically, finally opening the coffin and finding Sydney unconscious. He performs CPR. After a few tense moments, she opens her eyes. And exhausted Marshall drops to the ground.

In the office, Sloane reports that they have acquired information about Ulrich. Since an attack is imminent, Sydney needs to get to Ulrich, find his hard drive and upload the contents to APO so they can stop the detonation. Sydney responds by saying that Ulrich knows her from Havana, and that they need to send someone else in: Marshall.

Sydney advises Marshall on the spy game. Marshall, assuming an intimidating pose as Jack Bristow, walks inside Ulrich's private room and commands his attention. He tells Ulrich that one of his men is betraying him. As his "cigarette case" scans Ulrich's laptop, Vaughn reports from APO that they still can't access the information, and that Marshall needs to go to the basement with Ulrich in order to force him to shut down the network firewall. As Marshall tries to subtly assemble the gun he brought, he accidentally shoots and kills Ulrich. He calls Sydney for help.

From the APO office, Jack instructs Marshall to dig out one of Ulrich's eyes since the security in the basement works on a retinal scan. Sydney arrives and they bypass the basement system security using Ulrich's eye. Marshall then starts downloading from the hard drive and transmits intel to APO while Sydney covers him. Later, Sloane reports that the Ministry of Security confirmed the seizure of the Cyclosarin bomb in downtown Hong Kong.

Sydney drops Marshall off at his house and he invites her in for dinner. A grateful Sydney refuses, saying that she needs to go the train station. At the station, Vaughn arrives. They slow dance in the middle of the train station, blissfully unaware of the bustling world around them.